Such a mobile communication system is already known in the art, e.g. from the articles "ECR900 Digital Cellular Mobile Radio System" by M. Ballard and D. Verhulst, Electrical Communication, Vol. 63, No. 1, 1989, pp. 45-51, and "Cellular Mobile Radio as an Intelligent Network Application" by M. Ballard, E. Issenmann and M. Sanchez, Electrical Communication, Vol. 63, No. 4, 1989, pp. 389-399. The time division multiple access (TDMA) structure used in this system is well known in the art and described e.g. in the book "Digital Communications--fundamentals and applications" by B. Sklar, published by Prentice-Hall International Editions, 1988, pp. 484-486. After the transmission of the above request for access or service request, an authentication procedure is started to prevent fraudulent call attempts or listening-in by third parties. Such an authentication procedure is described e.g. in the second mentioned article. After authentication the base station assigns to the mobile station a traffic channel via which further communication can take place.
The above known system uses a constant envelope modulation technique, viz. Gaussian Minimum-shift-keying (GMSK) as explained in the article "Advanced VLSI Components for Digital Cellular Mobile Radio" by M. Rahier, D. Rabaey and J. Dulongpont, Electrical Communication, Vol. 63, No. 4, 1989, p. 410. A drawback of such a constant envelope modulation scheme is that it does not provide an optimal spectral efficiency, in contrast with a so-called linear modulation scheme, as is explained for example in the book "Digital cellular radio" by G. Calhoun, published by Artech House, Inc., 1988, p. 317. On the other hand, a drawback of a linear modulation scheme is that it provides signals with a non-constant envelope and which, to avoid spreading of the transmission spectrum, should only be amplified by linear amplifiers, as argued e.g. in the book "Digital Communications: satellite/earth station engineering" by K. Feher, published by Prentice-Hall Inc., 1983, p. 199. Although amplifiers used for amplifying constant envelope modulated signals theoretically do not have to operate in a linear way, in practice the envelope of the latter signals is not a constant so that some linearity is required.
If two (or more) mobile subscriber stations transmit requests for access which are modulated in the same access channel, i.e. on a same access carrier wave, and within the same time slot, collision occurs. Due to the superposition of these requests the base station cannot distinguish between the requesting subscriber stations, but detects the collision. The base station then launches a so-called negative acknowledgement to the colliding mobile subscriber stations indicating their request for access has not been received in good order, and so giving rise to the launching of another request for access by the mobile subscriber stations. Although the chance for a new collision is small since this second request for access occurs after a random delay, the setting up of a communication can be strongly delayed, especially under heavy traffic conditions, i.e. the more mobile subscriber stations want to set up a communication per unit of time, the longer the average delay becomes. Since collision can only occur when two or more subscribers request access to the base station within a same time slot, reducing the time slot length reduces the probability for collision. However, several factors require a minimum time slot length. Firstly, as explained above, the amplifiers amplifying modulated signals need some time, called ramp up time, to enable them to operate in a linear way and without giving rise to a large spectrum of noise. Secondly, and for the same reason, they also need some time, called ramp down time, to be gradually cut off. Finally, a small so-called guard time is also needed to prevent interference between adjacent time slots due to differences in time delay between different subscriber stations and the base station. However, the guard time can be very small, depending on the distances between the different subscribers and the base station, i.e. depending on the area covered by the base station.
From the above it follows that a lower limit for the time slot length is required mainly because of ramp up and ramp down times. Moreover, since within these times no useful information is communicated, i.e. since these times are to be considered as idle times, they decrease the information rate of the communication. Therefore and because these times do not depend on the time slot length a minimum required information rate imposes a further constraint for minimizing the time slot length.